There are a variety of processes that employ adsorbents that are capable of adsorbing one or more impurities contained within a feed stream more readily than other impurities within the feed stream to produce a product stream having a lower concentration of the impurities than the feed stream. The adsorbent is contained within an adsorption bed and one or more adsorption beds can be utilized in such processes. For example, such process can be used to purify a hydrogen containing stream from a reformer. In such case, water vapor, carbon dioxide, carbon monoxide, other hydrocarbons and nitrogen are present within the feed stream. The hydrogen is the less readily adsorbed component and as such constitutes the product stream. The impurities or components, water vapor, carbon dioxide and etc. are the more readily adsorbed components and are removed from the feed stream through adsorption to produce the product stream. The adsorbent or adsorbents are located within an adsorbent bed that can consist of a vessel having one or more layers of adsorbent. In case of a hydrogen containing stream, the adsorbent bed could be provided with an initial layer of alumina to adsorb the water vapor, a layer of activated carbon to adsorb the carbon dioxide and heavier hydrocarbons and a final layer of a zeolite adsorbent to adsorb the carbon monoxide and nitrogen.
In any adsorption process, a time is reached at which the adsorbent is fully laden with the impurity or impurities and the adsorbent bed must be regenerated. As such, adsorption processes employ a cycle during which an adsorption bed is on-line and adsorbing the impurities or components and then, is subsequently off-line and being regenerated. Adsorption processes can be distinguished by the cycle employed, for example, pressure swing adsorption, temperature swing adsorption and vacuum pressure swing adsorption. In the example given above regarding the production of a hydrogen product stream, the cycle employed is pressure swing adsorption.
In pressure swing adsorption two or more adsorbent vessels containing the adsorbent beds are employed in an out of phase cycle so that while one bed is adsorbing the impurities, another or other beds can be regenerated. A pressure swing adsorption cycle can have the elements of adsorption in which it is supplying a product, depressurization by way of one or more equalization steps followed by a step in which the bed provides purge gas to another bed to cause desorption of adsorbed components from the bed. A blow down step in which the inlet end of the bed is opened to discharge adsorbed components followed by a purge step with purge gas provided by another bed to further desorb components from the adsorbent of the adsorbent bed. This is followed by one or more equalization steps in which the adsorbent bed is partially repressurized and a product repressurization step in which the adsorbent bed is fully repressurized and able to be brought back on line. Each adsorbent bed is subjected to all elements of the cycle and as such product is continually being delivered. In a temperature swing adsorption process, the adsorbent bed is regenerated by heating the adsorbent bed to a high temperature with a hot gas to reduce the capacity of the adsorbent and thereby to cause the impurities to be desorbed. After having been heated, the adsorbent bed is cooled prior to being brought back on-line. In vacuum pressure swing adsorption, an adsorbent bed is at least in part regenerated under vacuum. For example, an adsorbent that will readily adsorb nitrogen, carbon dioxide and water vapor is used in such a cycle to produce oxygen. Such a process can utilize a single adsorption bed and therefore, the product is discharged into a surge tank so that produce may be continually produced. Multiple adsorbent beds can also be used in such a process to produce the product at a higher rate than a single adsorbent bed.
In any such adsorption process the adsorbent bed or beds are connected to a flow control network having valves to subject the beds to the various steps of the particular cycle. The valves within the flow control network for an adsorption unit conducting the process discussed above, or other process are controlled to open and close to subject each of the adsorbent beds to the production and regeneration over prespecified time periods. As known in the art, however, the feed to the adsorbent beds is subject to such upsets as flow, concentration and temperature variations that can result in the product stream not meeting a product specification or in other words, having product impurity concentrations that are at too high a level. It is known that a primary means for controlling product purity is to adjust the feed cycle time that each adsorbent bed spends in the adsorption step. If the product impurity concentration within the product stream is too high, the feed cycle time is shortened. On the other hand, if the product impurity concentration is below a target, the feed cycle time is lengthened to in turn increase the production of the product. Conventionally, the operator monitors the product impurity concentration and then manually adjusts the feed cycle time. This can be automated through a feedback control system. In both cases, however, there will be a lag in a change in the product purity following an upset or in other words, the change in purity upon the occurrence of an upset will not be instantaneous. As such, in case of either manual or automated control, the target at which a control action is taken will be selected so that the product will never exceed the product specification. The result of this is that the feed cycle time on average will be shorter than would be required to actually meet the product specification and therefore, the production rate of the product will be less than could otherwise be obtained. Feed forward control systems have also been used in which the feed composition and flow are measured on-line. A major problem with such a system is that a model or models must be used to gauge the effect of such changes in the feed on the product purity are not perfect and therefore, again, the targets will be conservatively selected with the result of lower production rates.
U.S. Pat. No. 4,693,730 provides a control system in which a characteristic of the effluent from an adsorbent bed undergoing a depressurization is sensed and then a corrective action is taken in response to the sensed characteristic. For example, the sensed characteristic could be the impurity level in the effluent and the corrective action could be to change the feed cycle time. In the control system contemplated in this patent, the effluent concentration is compared to a target. After the target has been reached, if the product gas impurity level is not at the desired value, an error between the actual and desired value is used to calculate a new target value. However, such effluent concentration will not experience an instantaneous change upon an upset and therefore, the target selected will be a conservative target.
U.S. Pat. No. 7,025,801 discloses a control method for a pressure swing adsorption unit in which the flow rate of the feed stream is monitored and upon an increase in flow rate that would tend to drive the product purity off its specification, the cycle time is reduced and vice-versa. Additionally, the purity of the product stream is also measured. Upon an increase in product purity above the product specification, the cycle time is reduced and vice-versa. Upon an increase in concentration of the impurity within the feed stream without an increase in flow rate and upon data from a controller indicative of such event, the cycle time is adjusted and possibly other steps within the pressure swing adsorption cycle. Thus, the method of this patent contemplates both feed back and feed forward control having the drawbacks outlined above.
As will be discussed, among other advantages, the present invention provides a control method and system for controlling an adsorption unit in which impurity concentration is sensed within the adsorbent bed itself rather than in a feed stream, a product stream or an effluent stream. The change in such impurity concentration upon an upset will be more rapid than in the feed, effluent or product. As a result, targets are able to be selected that are less conservative than in the prior art resulting in longer cycle times and higher production rates.